ia64/linux-2.6.18-xen.hg

view Documentation/i2c/dev-interface @ 854:950b9eb27661

usbback: fix urb interval value for interrupt urbs.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Apr 06 13:51:20 2009 +0100 (2009-04-06)
parents 831230e53067
children
line source
1 Usually, i2c devices are controlled by a kernel driver. But it is also
2 possible to access all devices on an adapter from userspace, through
3 the /dev interface. You need to load module i2c-dev for this.
5 Each registered i2c adapter gets a number, counting from 0. You can
6 examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
7 I2C device files are character device files with major device number 89
8 and a minor device number corresponding to the number assigned as
9 explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
10 i2c-10, ...). All 256 minor device numbers are reserved for i2c.
13 C example
14 =========
16 So let's say you want to access an i2c adapter from a C program. The
17 first thing to do is "#include <linux/i2c-dev.h>". Please note that
18 there are two files named "i2c-dev.h" out there, one is distributed
19 with the Linux kernel and is meant to be included from kernel
20 driver code, the other one is distributed with lm_sensors and is
21 meant to be included from user-space programs. You obviously want
22 the second one here.
24 Now, you have to decide which adapter you want to access. You should
25 inspect /sys/class/i2c-dev/ to decide this. Adapter numbers are assigned
26 somewhat dynamically, so you can not even assume /dev/i2c-0 is the
27 first adapter.
29 Next thing, open the device file, as follows:
30 int file;
31 int adapter_nr = 2; /* probably dynamically determined */
32 char filename[20];
34 sprintf(filename,"/dev/i2c-%d",adapter_nr);
35 if ((file = open(filename,O_RDWR)) < 0) {
36 /* ERROR HANDLING; you can check errno to see what went wrong */
37 exit(1);
38 }
40 When you have opened the device, you must specify with what device
41 address you want to communicate:
42 int addr = 0x40; /* The I2C address */
43 if (ioctl(file,I2C_SLAVE,addr) < 0) {
44 /* ERROR HANDLING; you can check errno to see what went wrong */
45 exit(1);
46 }
48 Well, you are all set up now. You can now use SMBus commands or plain
49 I2C to communicate with your device. SMBus commands are preferred if
50 the device supports them. Both are illustrated below.
51 __u8 register = 0x10; /* Device register to access */
52 __s32 res;
53 char buf[10];
54 /* Using SMBus commands */
55 res = i2c_smbus_read_word_data(file,register);
56 if (res < 0) {
57 /* ERROR HANDLING: i2c transaction failed */
58 } else {
59 /* res contains the read word */
60 }
61 /* Using I2C Write, equivalent of
62 i2c_smbus_write_word_data(file,register,0x6543) */
63 buf[0] = register;
64 buf[1] = 0x43;
65 buf[2] = 0x65;
66 if ( write(file,buf,3) != 3) {
67 /* ERROR HANDLING: i2c transaction failed */
68 }
69 /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
70 if (read(file,buf,1) != 1) {
71 /* ERROR HANDLING: i2c transaction failed */
72 } else {
73 /* buf[0] contains the read byte */
74 }
76 IMPORTANT: because of the use of inline functions, you *have* to use
77 '-O' or some variation when you compile your program!
80 Full interface description
81 ==========================
83 The following IOCTLs are defined and fully supported
84 (see also i2c-dev.h):
86 ioctl(file,I2C_SLAVE,long addr)
87 Change slave address. The address is passed in the 7 lower bits of the
88 argument (except for 10 bit addresses, passed in the 10 lower bits in this
89 case).
91 ioctl(file,I2C_TENBIT,long select)
92 Selects ten bit addresses if select not equals 0, selects normal 7 bit
93 addresses if select equals 0. Default 0.
95 ioctl(file,I2C_PEC,long select)
96 Selects SMBus PEC (packet error checking) generation and verification
97 if select not equals 0, disables if select equals 0. Default 0.
98 Used only for SMBus transactions.
100 ioctl(file,I2C_FUNCS,unsigned long *funcs)
101 Gets the adapter functionality and puts it in *funcs.
103 ioctl(file,I2C_RDWR,struct i2c_rdwr_ioctl_data *msgset)
105 Do combined read/write transaction without stop in between.
106 The argument is a pointer to a struct i2c_rdwr_ioctl_data {
108 struct i2c_msg *msgs; /* ptr to array of simple messages */
109 int nmsgs; /* number of messages to exchange */
110 }
112 The msgs[] themselves contain further pointers into data buffers.
113 The function will write or read data to or from that buffers depending
114 on whether the I2C_M_RD flag is set in a particular message or not.
115 The slave address and whether to use ten bit address mode has to be
116 set in each message, overriding the values set with the above ioctl's.
119 Other values are NOT supported at this moment, except for I2C_SMBUS,
120 which you should never directly call; instead, use the access functions
121 below.
123 You can do plain i2c transactions by using read(2) and write(2) calls.
124 You do not need to pass the address byte; instead, set it through
125 ioctl I2C_SLAVE before you try to access the device.
127 You can do SMBus level transactions (see documentation file smbus-protocol
128 for details) through the following functions:
129 __s32 i2c_smbus_write_quick(int file, __u8 value);
130 __s32 i2c_smbus_read_byte(int file);
131 __s32 i2c_smbus_write_byte(int file, __u8 value);
132 __s32 i2c_smbus_read_byte_data(int file, __u8 command);
133 __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
134 __s32 i2c_smbus_read_word_data(int file, __u8 command);
135 __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
136 __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
137 __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
138 __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,
139 __u8 *values);
140 All these transactions return -1 on failure; you can read errno to see
141 what happened. The 'write' transactions return 0 on success; the
142 'read' transactions return the read value, except for read_block, which
143 returns the number of values read. The block buffers need not be longer
144 than 32 bytes.
146 The above functions are all macros, that resolve to calls to the
147 i2c_smbus_access function, that on its turn calls a specific ioctl
148 with the data in a specific format. Read the source code if you
149 want to know what happens behind the screens.